Foams to be used as internal insulators for electronic appliances or the like, cushioning materials, sound insulators, heat insulators, food packaging materials, clothing materials, building materials, and the like are required to have properties such as softness, cushioning properties, and heat-insulating properties from the standpoint of securing the sealing and other properties of such foams in the case where the foams are incorporated as parts.
These foams are further required to have a given thickness so as to retain sealing properties. Known as such foams are polyolefin resin foam such as polyethylene and polypropylene foams. However, these foams have had a problem that they have a poor strength and are insufficient also in softness and cushioning property. Techniques for improving these insufficient properties are being employed, which are to heighten the expansion ratio and to incorporate a rubber ingredient or the like into a polyolefin resin to soften the material itself.
However, since ordinary polyethylene or polypropylene is low in the viscosity relating to the stress required for high-temperature stretching, i.e., extensional viscosity, a foaming operation conducted so as to obtain a high expansion ratio results in cell wall breakage and hence in gas escape and cell enlargement. It has therefore been difficult to obtain a soft foam having the desired high expansion ratio.
Known processes heretofore in use for producing a polymer foam include a chemical process and a physical process. The general physical process comprises dispersing a low-boiling liquid (blowing agent) such as a chlorofluorocarbon or hydrocarbon in a polymer and then heating the dispersion to volatilize the blowing agent and thereby form cells. The chemical process for obtaining a foam comprises pyrolyzing a compound (blowing agent) added to a polymer base to generate a gas and thereby form cells. However, the technique of physical foaming has various environmental problems such as the harmfulness of the substance used as a blowing agent and ozonosphere depletion caused by the substance, while the technique of chemical foaming has a problem that a residue of the blowing agent remains after gas generation in the foam and this residue is causative of pollution especially in applications such as electronic parts.
On the other hand, a process for obtaining a foam having a small cell diameter and a high cell density has recently been proposed. This process comprises dissolving a gas such as nitrogen or carbon dioxide in a polymer at a high pressure, subsequently releasing the polymer from the pressure, and heating the polymer to a temperature around the glass transition temperature or softening point of the polymer to thereby form cells. In this foaming technique, nuclei are formed in a system in a thermodynamically unstable state and these nuclei expand and grow to form cells and thereby give a micro-cellular foam.
This process has an advantage that a foam having a micro-cellular structure which has not been obtained so far can be produced. Various attempts to apply this foaming technique to thermoplastic elastomers including thermoplastic polyurethanes have been proposed. However, a sufficiently high expansion ratio has not been obtained and the foams obtainable with the technique have been limited to thin foams.
For overcoming those problems, a polyolefin resin foam made of a blend of a polyolefin resin having, for example, a melt tension exceeding 1 cN at 230° C. and a rubber or thermoplastic elastomer ingredient and a process for foam production using carbon dioxide in a supercritical state have been proposed (patent document 1). Furthermore, it has been disclosed that a polystyrene composition having specific values of extensional viscosity, weight-average molecular weight, etc. has excellent suitability for foaming (patent document 2).
[Patent Document 1]
JP-A-2001-348452 (Claims)
[Patent Document 2]
JP-A-09-208771 (Claims)